A microscope apparatus in which a laser is used as an irradiation light source and a laser beam outputted from an irradiation light source is irradiated onto a sample in order to observe the sample has been conventionally used. In such a microscope, in order to increase resolution, it is desirable that the diameter of beam spot radiated from the irradiation light source and focused onto a sample be as small as possible. In general, a minimum diameter of a beam spot is defined by diffraction limit, and the minimum diameter is thus proportional to wavelength of light. Therefore, the shorter the wavelength of light radiated from a light source, the smaller the diameter of a beam spot.
However, a light source that emits short wavelength light, i.e., a laser emitting violet light or ultra violet light is generally more expensive than a light source that emits light of longer wavelength, for example green light or red light. Also, in general, in the wavelength range from violet to ultra violet, light transmittance of optical material decreases as wavelength becomes shorter. Therefore, an optical material that exhibits high light transmittance for violet or ultra violet light is very limited.
Thus, radial polarization is now attracting much attention. Radial polarization is a polarized light beam in which polarization plane of linearly polarized light is distributed radially with the optical axis as a center. It is reported that, by focusing a light beam of radial polarization with a condenser to a focus, the focused light beam in the focus plane has Z-polarization (i.e., the direction of electric field is same as the direction of propagation of light), so that light beam can be focused in smaller spot diameter than the spot diameter defined by diffraction limit of X- or Y-polarization). It is also reported that as the range of a small beam diameter along the optical axis becomes longer, and larger depth of focus can be achieved. (See, for example, Chin-Cherng Sun, Chin-Ku Liu, “Ultrasmall focusing spot with a long depth of focus based on polarization and phase modulation”, OPTICS LETTERS, Optical Society of America, 2003, vol. 28, No. 2, p. 99-101; and HAIFENG WANG, et. al., “Creation of a needle of longitudinally polarized light in vacuum using binary optics”, Nature photonics, 2008, vol. 2, p. 501-505).